The blue field entoptic phenomenon is an

entoptic phenomenon Entoptic phenomena () are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In Helmholtz's words: "Under suitable conditions light falling o ...

characterized by the appearance of tiny bright dots (nicknamed blue-sky sprites) moving quickly along squiggly lines in the

visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...

, especially when looking into bright

blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...

light such as the

sky The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space. In the field of astronomy, ...

. The dots are short-lived, visible for a second or less, and traveling short distances along seemingly random, curvy paths. Some of them follow the same path as predecessors. The dots may be elongated along the path like tiny worms. The speed of the dots varies in sync with the

pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...

; they briefly accelerate at each heartbeat. The dots appear in the central

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans a ...

, within 15 degrees from the fixation point. The left and right eye see different dots; someone looking with both eyes sees a mixture. Most people are able to see this phenomenon in the sky, although it is rather weak, and many people do not notice it until asked to pay attention. The dots are highly conspicuous against a

monochromatic A monochrome or monochromatic image, object or color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...

blue background (~430 nm) instead of the sky. The phenomenon is also known as Scheerer's phenomenon after the German

ophthalmologist Ophthalmology ( ) is a surgery, surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Followin ...

Richard Scheerer, who first drew clinical attention to it in 1924.

Explanation

The dots are

white blood cell White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...

s moving in the

capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

in front of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

of the eye. Blue light (optimal

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

: 430 nm) is absorbed by the

red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

s that fill the capillaries. The eye and brain "edit out" the shadow lines of the capillaries, partially by

dark adaptation Darkness, the direct opposite of lightness, is defined as a lack of illumination, an absence of visible light, or a surface that absorbs light, such as black or brown. Human vision is unable to distinguish colors in conditions of very low l ...

of the photoreceptors lying beneath the capillaries. The white blood cells, which are larger than red blood cells, but much rarer and do not absorb blue light, create gaps in the blood column, and these gaps appear as bright dots. The gaps are elongated because a spherical white blood cell is too wide for the capillary. Red blood cells pile up behind the white blood cell, showing up like a dark tail. This behavior of the blood cells in the capillaries of the retina has been directly observed in human subjects by

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

scanning laser ophthalmoscopy Scanning laser ophthalmoscopy (SLO) is a method of examination of the eye. It uses the technique of confocal laser scanning microscopy for diagnostic imaging of the retina or cornea of the human eye. As a method used to image the retina with a h ...

, a real time imaging technique for examining retinal blood flow. The dots will not appear at the very center of the visual field, because there are no blood vessels in the

fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...

l avascular zone.

Blue field entoptoscopy

In a technique known as blue field entoptoscopy, the effect is used to estimate the blood flow in the retinal capillaries. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture to the perceived entoptic dots.

Difference from other entoptic phenomena

Scheerer's phenomenon can be easily distinguished from

floater Floaters or eye floaters are sometimes visible deposits (e.g., the shadows of tiny structures of protein or other cell debris projected onto the retina) within the eye's vitreous humour ("the vitreous"), which is normally transparent, or betwee ...

s (''muscae volitantes''). Scheerer's phenomenon consists of corpuscles of identical diameter and visual sharpness, of a simple dot or worm-like shape, brighter than the background. If the eye stops moving, the dots keep darting around. If the eye moves, the dots follow instantaneously, because they are contained in the retina. In contrast, floaters are specks or threads of variable diameter and variable visual sharpness, some of complex shape, darker than the background. If the eye stops moving, the floaters settle down. If the eye moves, the floaters follow sluggishly, because they are contained in the

vitreous humor The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humor ...

, which is subject to inertia. Scheerer's phenomenon can be distinguished from

visual snow Visual snow syndrome (VSS) is an uncommon neurological condition in which the primary symptom is that affected individuals see persistent flickering white, black, transparent, or coloured dots across the whole visual field. Other common symptom ...

because it appears only when looking into bright light, whereas visual snow is constantly present in all light conditions including the dark.

References

External links

Video describing history and science of blue field entoptic phenomenon and viewing techniques
{{Visual phenomena Optical phenomena Ophthalmology Vision

Explanation

Blue field entoptoscopy

Difference from other entoptic phenomena

See also

References

External links